But this is the first study to compare their rates of health issues to those of U.S. children as a whole, including kids from low-income or single-parent families.

The study confirms that children in foster care are particularly vulnerable, said lead researcher Kristin Turney, an associate professor of sociology at the University of California, Irvine.

Overall, her team found, foster kids faced two to three times higher risks of physical health issues such as asthma, obesity and hearing and vision problems.

And they were five to seven times more likely to have behavioral issues or symptoms of depression or anxiety.

No one is saying that foster care caused those problems, Turney stressed. "We can't conclude that foster care created them, or made them worse," she said.

The point, according to Turney, was to paint a clearer picture of how U.S. foster children are faring.

Under the foster care system, a designated adult temporarily cares for a child or children whose birth parent is unable to care for them.

To Julie Steele, the results of the new study ring true to what she sees in her own practice.

Steele is an assistant professor of nursing at the University of Utah, where she also manages the South Main Foster Care Clinic.

She said that while the study findings aren't surprising, they're important. "They expand our understanding, by really quantifying the risks these children face," said Steele, who was not involved in the research.

Beyond that, she said, the study shines a light on a group of children who often "fall below the radar."

In general, Steele said, children should get physical and mental health evaluations when they enter the foster system. And ideally, she added, kids who need help with problems like depression and anxiety should get specialized "trauma-informed" therapy.

But the reality does not always match the ideal.

First, Steele explained, foster care systems and the laws governing them vary by state. Local resources also vary: In rural areas, for example, there may be no providers with expertise in trauma-informed therapy, she pointed out.

What's needed, Steele said, is better funding and more resources. "As a society, we need to support these vulnerable children," she said.

The findings come from a nationally representative government health survey done between 2011 and 2012. It covered nearly 96,000 children and teenagers, 0.5 percent of whom were in foster care.

Overall, foster care kids had higher rates of certain physical health issues: 18 percent had asthma, for example, versus 9 percent of other U.S. kids. Almost one-quarter were obese, versus 16 percent of other children.

Often, the study found, there were more stark differences when it came to mental health and behavior.

Between 14 percent and 17 percent of foster children had been diagnosed with anxiety, depression or behavior problems, based on foster parents' reports. That compared with 2 percent to 3 percent of other U.S. kids. Another 22 percent had been diagnosed with ADHD, versus about 7 percent of other kids.

The findings were published online Oct. 17 in the journal Pediatrics.

Turney pointed to another important finding. Foster children had higher risks of mental and behavioral issues -- even when compared with other kids who often face challenges, like those from low-income or single-parent families.

For foster parents, Steele said, the message is to ask for help when they need it.

"They can reach out to health care providers," she said. "They should not feel like they have to do it all themselves."